The invention relates generally to measurement of lung volume at different inspiration and expiration pressures during artificial ventilation of humans or other mammals. More particularly, this invention relates to the utilization of those measurements to determine the optimum range of pressures for artificial ventilation.
Frequently, a sick patient must be assisted in breathing by a ventilator. This patient may be human, or a nonhuman mammal. During conventional mechanical ventilation (CMV), the lung is inflated with a distending pressure called positive end expiratory pressure (PEEP). During high frequency ventilation (HFV), the lung is inflated with a distending pressure called mean airway pressure (Paw).
In a diseased lung, some air sacs may collapse, preventing gas from entering or leaving and thereby preventing gas exchange through those air sacs. Because a fewer number of air sacs are available for gas exchange, the patient must be ventilated with a higher concentration of oxygen than normal to enable his or her remaining open air sacs to provide adequate blood oxygenation. While a high oxygen concentration is required to provide adequate blood oxygenation and keep the patient alive, it is also toxic.
During inflation of the lung with increasing PEEP or Paw, the pressure increases and the collapsed air sacs of the lung begin to open, allowing them to once again take part in gas exchange. The pressure at which the air sacs begin to open is called the critical opening pressure. Air sacs opened because of ventilator inflation pressure are said to be recruited. As the number of recruited air sacs increases, the amount of oxygen that diffuses into the arterial blood also increases. This is reflected as an increase in blood oxygen saturation level, as may be non-invasively measured by pulse oximetry or directly with an arterial blood gas measurement. The increase in oxygen in the blood enables the caregiver to lower the inspired oxygen concentration toward less toxic levels. For these reasons, it is generally beneficial to recruit as many air sacs as possible in a patient undergoing ventilation.
When an increase in ventilator pressure fails to improve the oxygen saturation level of the blood, the lung is considered to be stable. If additional pressure is added to a stable lung, the patient runs the risk of experiencing overinflation. Overinflation significantly increases the chances for lesions to form in the lung tissue. Such lesions can allow air to leak into the space between the lungs and the chest wall, and can be lethal.
If the lung has been pressurized to the point of overinflation during recruitment, pressure cannot simply be reduced incrementally. Due to the elasticity of the lungs, which causes a nonlinear pressure/volume relationship which is different for inhalation than exhalation, a pressure decrease does not lead to a significant lung volume decrease until decruitment of air sacs begins. This pressure at which the air sacs begin to derecruit is called the critical closing pressure. A simple pressure decrease therefore leaves the lung in the same dangerous overinflated condition. Consequently, to prevent overinflation in a given patient, ventilator pressures may be significantly reduced to a lower level to find the safest pressure with the air sacs recruited. However, when the pressure is significantly reduced, the patient's blood oxygen can fall to dangerously low levels due to derecruitment of air sacs. As can be seen, this trial and error method is risky for the ventilated patient.
Underinflation of the lung creates another set of physical problems. If the lung is underinflated, diseased lung tissue may be derecruited, causing a condition called atelectasis. That is, diseased air sacs that took part in gas exchange when the lung was properly inflated (i.e., air sacs that had been recruited) will no longer do so if the inflation pressure is too low. Those air sacs will close again, and no gas exchange will take place through them, inhibiting the patient's ability to absorb oxygen and jettison carbon dioxide. Underinflation thereby causes atelectasis, which may be a life-threatening condition. Finally, underinflation can result in the release of chemicals in the lung tissue that induce biochemical lung injury.
Given the dangers of overinflation and underflation, the pressure output of a ventilator must be high enough to prevent underflation, and low enough to prevent overinflation. In the case of HFV, the mean airway pressure (Paw) must fall in this range between underinflation and overinflation. In the case of CMV, PEEP must fall in this range.
Currently, frequent chest X-rays are used clinically to determine an appropriate inflation pressure and to detect over- or under-inflation of the lungs. These frequent X-rays are undesirable for several reasons. First, they are clinically impractical when making frequent adjustments in ventilator settings. Second, they expose the patient to a significant cumulative dose of X-ray radiation during the course of ventilation. Third, accurately determining lung volume from a chest X-ray is difficult at best. Finally, frequent X-rays are costly to perform.